Megakaryocyte structure and function

Distinct functional effects for dynamin 3 during megakaryocytopoiesis

Dynamin 3 (DNM3) is a member of a family of motor proteins that participate in a number of membrane rearrangements such as cytokinesis, budding of transport vesicles, phagocytosis, and cell motility. Recently, DNM3 was implicated as having a role in megakaryocyte (MK) development. To further investigate the functional role of DNM3 during megakaryocytopoiesis, we introduced sequence-specific short hairpin RNAs (shRNAs) into developing MKs. The results showed that knockdown of DNM3 inhibited a stage of MK development that involved progenitor amplification. This was evident by significant decreases in the number of colony forming unit-megakaryocytes, the total number of nucleated cells, and the number of CD41(+) and CD61(+) MKs produced in culture. Using a styrl membrane dye to quantify the demarcation membrane system (DMS) of terminally differentiated MKs, we found that DNM3 co-localized with the DMS and that DNM3 lentiviral shRNAs precluded the formation of the DMS. Knockdown of dynamin 3 in murine MKs also caused a decrease in the number of morphologically large MKs and the overall size of large MKs was decreased relative to controls. MK protein lysates were used in overlay blots to show that both DNM3 and actin bind to nonmuscle myosin IIA (MYH9). Consistent with these observations, immunofluorescence studies of MKs and proplatelet processes showed co-localization of DNM3 with MYH9. Overall, these studies demonstrate that DNM3 not only participates in MK progenitor amplification, but is also involved in cytoplasmic enlargement and the formation of the DMS.

Thrombopoietin-induced Dami cells as a model for α-granule biogenesis

Megakaryocytic alpha-granules contain secretory proteins relevant to megakaryocyte and platelet functions. Understanding alpha-granule biogenesis is hampered because human primary megakaryocytes are difficult to manipulate. Existing promegakaryocytic cell lines do not spontaneously exhibit mature alpha-granules. Dami cells, transfected with the megakaryocytic platelet factor 4, fused to GFP (PF4-GFP), were induced in the presence of thrombopoietin (TPO), a megakaryocyte cytokine and PMA. Using confocal microscopy, PF4-GFP colocalized with von Willebrand Factor (vWF) in newly formed storage granules. Immunoelectron microscopy demonstrated alpha-granule-like features, including a dense core or parallel tubules and colocalization of PF4-GFP and vWF. Hence, TPO-treated Dami cells are a suitable model to study alpha-granules and their biogenesis.

Intrinsic impaired proplatelet formation and microtubule coil assembly of megakaryocytes in a mouse model of Bernard-Soulier syndrome

BACKGROUND

Giant platelets and thrombocytopenia are invariable defects in the Bernard-Soulier syndrome caused by deficiency of the GPIb-V-IX complex, a receptor for von Willebrand factor supporting platelet adhesion to the damaged arterial wall. Various properties of this receptor may be considered potential determinants of the macrothrombocytopenia.

DESIGN AND METHODS

To explore the underlying mechanisms of the disease, megakaryopoiesis was studied in a mouse model deficient in GPIbbeta. Megakaryocytes were initially characterized in situ in the bone marrow of adult mice, after which their capacity to differentiate into proplatelet-bearing cells was evaluated in cultured fetal liver cells.

RESULTS

The number of megakaryocyte progenitors, their differentiation and progressive maturation into distinct classes and their level of endoreplication were normal in GPIbbeta(-/-) bone marrow. However, the more mature cells exhibited ultrastructural anomalies with a thicker peripheral zone and a less well developed demarcation membrane system. GPIbbeta(-/-) megakaryocytes could be differentiated in culture from Lin(-) fetal liver cells in normal amounts but the proportion of cells able to extend proplatelets was decreased by 41%. Moreover, the GPIbbeta(-/-) cells extending proplatelets displayed an abnormal morphology characterized by fewer pseudopodial extensions with thicker shaft sections and an increased diameter of the terminal coiled elements. GPIbbeta(-/-) released platelets were larger but retained a typical discoid shape. Proplatelet formation was similarly affected in bone marrow explants from adult mice examined by videomicroscopy. The marginal microtubular ring contained twice as many tubulin fibers in GPIbbeta(-/-) proplatelet buds in cultured and circulating platelets.

CONCLUSIONS

Altogether, these findings point to a role of the GPIb-V-IX complex intrinsic to megakaryocytes at the stage of proplatelet formation and suggest a functional link with the underlying microtubular cytoskeleton in platelet biogenesis.

Abnormal megakaryocyte morphology and proplatelet formation in mice with megakaryocyte-restricted MYH9 inactivation

Mutations in the MYH9 gene encoding nonmuscle myosin IIA lead to macrothrombocytopenia as observed in MYH9-related disorders. We used mice with megakaryocyte-restricted MYH9 inactivation to explore the role of myosin in thrombopoiesis. In situ, bone marrow MYH9Delta megakaryocytes were irregularly shaped, appearing leaky with poorly defined limits. The demarcation membranes were abnormally organized and poorly developed, pointing to an insufficient reservoir for the future formation of platelets. The cytoskeletal-rich peripheral zone was lacking due to the absence of the myosin filament network that normally surrounds the granular zone in wild-type cells. In vitro studies of cultured cells showed that MYH9Delta megakaryocytes were unable to form stress fibers upon adhesion to collagen, suggesting that the leaky shape results from defects in internal tension and anchorage to the extracellular environment. Surprisingly, the proportion of cells extending proplatelets was increased in MYH9Delta megakaryocytes and the proplatelet buds were larger. Overall, this study provides evidence for a role of myosin in different steps of megakaryocyte development through its participation in the maintenance of cell shape, formation and organization of the demarcation membranes and the peripheral zone, anchorage to the extracellular matrix, and proplatelet formation.

Delivering new insight into the biology of megakaryopoiesis and thrombopoiesis

PURPOSE OF REVIEW

The aim of this review is to explore the state of the art knowledge on the cell biological and molecular pathways that regulate megakaryopoiesis and lead to platelet production.

RECENT FINDINGS

In the last 2 years there has been considerable progress in the elucidation of molecular mechanisms of megakaryocyte development and platelet biogenesis, driven by the application of modern molecular biology approaches to these specialized and unique cells. Studies have for the first time visualized endomitotic spindle dynamics, characterized the maturation of the demarcation membrane system, and delineated the mechanics of organelle transport and microtubule assembly in living megakaryocytes. The role of specific molecules in platelet production has been elucidated in greater detail by combining molecular studies with genetically engineered mice as well as embryonic cell culture systems.

SUMMARY

This review integrates the latest studies of megakaryocyte development into the molecular pathways that regulate megakaryopoiesis and thrombopoiesis. Decoding the pathways of megakaryopoiesis and platelet production should help revolutionize the management of thrombocytopenia and other platelet disorders.

Efficient in vitro megakaryocyte maturation using cytokine cocktails optimized by statistical experimental design

OBJECTIVE

A multi-step statistical strategy was applied to quantify individual and interactive effects of cytokines on megakaryopoiesis and to determine the concentration of the selected cytokines that optimize ex vivo megakaryocyte (MK) expansion, maturation, and platelet production in stromal- and serum-free conditions.

MATERIALS AND METHODS

Immature MK were first generated from human CD34(+)-enriched cord blood cells cultured for 7 days in conditions favoring MK commitment. Then, the effect of different combinations of cytokines at various concentrations on MK differentiation and platelet production was tested on the day-7 MK.

RESULTS

A large-scale screening of 13 cytokines in the presence of thrombopoietin (TPO) using Placket-Burman designs (PBD) was initially performed to identify stimulators of MK maturation. Afterwards, a statistical analysis of the two-level factorial designs revealed that in the presence of TPO, MK maturation was significantly stimulated by stem cell factor (SCF), interleukin (IL)-6, and IL-9, whereas Flt-3 ligand (FL) had a positive effect only on the expansion of MK progenitors. In contrast, erythropoietin (EPO) and IL-8 were inhibitors of MK maturation. A response surface methodology was then used to optimize the concentrations of the selected cytokines (TPO, SCF, IL-6, and IL-9) and defined a new cytokine cocktail that maximized MK expansion and maturation. Importantly, the increased MK output was accompanied by a very high MK purity ( approximately 90%). Another optimum was also found at a higher SCF concentration, which further improved MK expansion and maturation, but reduced MK purity.

CONCLUSION

These statistical methods provide an efficient tool to analyze complex systems of cytokines and to develop promising ex vivo MK culture systems for clinical applications.

Monster cells in malignant melanoma

Cells with significantly enlarged nuclei have been described in basal cell carcinomas, dermatofibromas, and pleomorphic fibromas, to name a few. These cells are typically visible using low power microscopy and have been termed "pleomorphic" or "monster cells." They have not been previously described in cutaneous melanomas. We sought to determine the prevalence of monster cells in otherwise conventional biopsies of primary cutaneous melanomas and its association with other histopathologic features of this malignancy. Ninety-nine superficial spreading melanomas, nodular melanomas, and acral lentiginous melanomas/lentigo malignas were retrospectively evaluated for the presence of monster cells, multinucleated giant cells, ulceration, inflammation, and depth of invasion (Breslow level). Thirteen cases of melanoma containing monster cells were found. A statistically significant association was noted between the presence of these cells, the histologic subtype of nodular melanoma (P = 0.0125), ulceration (P = 0.0127), the depth of invasion (P = 0.0103), and the presence of multinucleated giant cells (P = 0.0016). The finding of monster cells is not an uncommon occurrence and is seen more often in nodular melanomas.

Evidence for a granule targeting sequence within platelet factor 4

Platelets achieve bleeding arrest at sites of vascular injury via secretion of secretory proteins from their storage granules, termed alpha-granules. We have recently analyzed granule targeting of platelet factor 4 (PF4), a secretory alpha-granule chemokine, and demonstrated that PF4 alpha-granule storage relied upon determinants within PF4 mature sequence. To define these determinants, PF4 mutants fused to the fluorescent reporter protein green fluorescent protein were generated by progressive deletions and site-directed mutagenesis. They were then transfected in AtT20 cells and assessed for granule targeting by colocalization with ACTH-containing granules, using laser scanning confocal microscopy. This strategy identified the amino acid 41-50 (LIATLKNGRK) sequence as most critical for PF4 granule targeting and/or storage; its deletion from PF4 induced a marked decrease in granule storage (from 81 +/- 2% to 17 +/- 3%, p < or = 0.0001). Ala-scanning mutagenesis of LIATLKNGRK narrowed down the targeting motif to LKNG. A direct role for LKNG in alpha-granule targeting was confirmed in the thrombopoietin-induced human megakaryocytic Dami cells, in which the LKNG-green fluorescent protein chimera exhibited an 82.5 +/- 1.8% colocalization with the alpha-granule proteins von Willebrand factor and P-selectin. LKNG is poorly conserved within the chemokine family. However three-dimensional alignments of the human alpha-granule chemokines Nap-2 (neutrophil-activating peptide) and RANTES (Regulated upon Activation Normal T Cell Expressed and Secreted) with PF4 revealed that LKNG, a surface-exposed hydrophilic turn/loop, matched Nap-2 (LKDG) and RANTES (TRKN) peptides with similar features. Moreover Nap-2 and RANTES peptides exhibited the same alpha-granule targeting efficiency than LKNG. We therefore postulate that the three-dimensional and physicochemical characteristics of PF4 LKNG are of general relevance to alpha-granule targeting of chemokines and possibly of other alpha-granule proteins.

Probing platelet factor 4 alpha-granule targeting

The storage mechanism of endogenous secretory proteins in megakaryocyte alpha-granules is poorly understood. We have elected to study the granule storage of platelet factor 4 (PF4), a well-known platelet alpha-granule protein. The reporter protein green fluorescent protein (GFP), PF4, or PF4 fused to GFP (PF4-GFP), were transfected in the well-characterized mouse pituitary AtT20 cell line, and in the megakaryocytic leukemic DAMI cell line. These proteins were also transduced using a lentiviral vector, in human CD34+ cells differentiated into megakaryocytes in vitro. Intracellular localization of expressed proteins, and colocalization studies were achieved by laser scanning confocal microscopy and immuno-electronmicroscopy. In preliminary experiments, GFP, a non-secretory protein (no signal peptide), localized in the cytoplasm, while PF4-GFP colocalized with adrenocorticotropin hormone (ACTH)-containing granules in AtT20 cells. In the megakaryocytic DAMI cell line and in human megakaryocytes differentiated in vitro, PF4-GFP localized in alpha-granules along with the alpha granular protein von Willebrand factor (VWF). The signal peptide of PF4 was not sufficient to specify alpha-granule storage of PF4, since when PF4 signal peptide was fused to GFP (SP4-GFP), GFP was not stored into granules in spite of its efficient translocation to the ER-Golgi constitutive secretory pathway. We conclude that the PF4 storage pathway in alpha-granules is not a default pathway, but rather a regular granule storage pathway probably requiring specific sorting mechanisms. In addition PF4-GFP appears as an appropriate probe with which to analyze alpha-granule biogenesis and its alterations in the congenital defect gray platelet syndrome.

Properties of the demarcation membrane system in living rat megakaryocytes

The demarcation membrane system (DMS) is the precursor of platelet cell membranes yet little is known of its properties in living megakaryocytes. Using confocal microscopy, we now demonstrate that demarcation membranes in freshly isolated rat marrow megakaryocytes are rapidly stained by styryl membrane indicators such as di-8-ANEPPS and FM 2-10, confirming that they are invaginations of the plasma membrane and readily accessible from the extracellular space. Two-photon excitation of an extracellular indicator displayed the extensive nature of the channels formed by the DMS throughout the extranuclear volume. Under whole-cell patch clamp, the DMS is electrophysiologically contiguous with the peripheral plasma membrane such that a single capacitative component can account for the biophysical properties of all surface-connected membranes in the majority of recordings. Megakaryocyte capacitances were in the range of 64-694 pF, equivalent to 500-5500 platelets (mean value 1850). Based upon calculations for a spherical geometry, the DMS results in a 4- to 14-fold (average 8.1-fold) increase in specific membrane capacitance expressed per unit spherical surface area. This indicates a level of plasma membrane invagination comparable with mammalian skeletal muscle. Whole-cell capacitance measurements and confocal imaging of membrane-impermeant fluorescent indicators therefore represent novel approaches to monitor the DMS during megakaryocytopoiesis and thrombopoiesis.

Identification of PML oncogenic domains (PODs) in human megakaryocytes

Megakaryocytes (Mks) are unique cells in the human body in that they carry a single and polyploid nucleus. It is therefore of interest to understand their nuclear ultrastructure. PML oncogenic domains (PODs) were described in several types of eukaryotic cells using human autoantibodies which recognize nuclear antigens with a specific speckled pattern (dots) in indirect immunofluorescence (IF). Two main antigens, PML and Sp 100, usually colocalize and concentrate in these nuclear subdomains. We investigated the presence of PODs using IF and immunoelectron microscopy (IEM) in cells from megakaryocytic lineage: the HEL cell line and human cultured Mks. Antibodies against PML, Sp100, and anti-nuclear dots were used in single and double labeling. PODs were identified in HEL cells and in human Mks, and their ultrastructure was characterized. We then used IF to quantify PODs within Mks and showed that their number increased proportionally to nuclear lobularity. In summary, we report the identification of PODs in human Mks at an ultrastructural level and an increase in PODs number in parallel with Mk ploidy. We show that endomitosis not only leads to DNA increase but also to the multiplication of at least one of the associated nuclear structures.

CD9 and megakaryocyte differentiation

It is shown that the tetraspanin CD9 has a complex pattern of distribution in hematopoietic cells and is heterogeneously expressed on human bone marrow CD34(+) cells. CD34(high)CD38(low)Thy1(+) primitive progenitors are contained in the population with intermediate CD9 expression, thus suggesting that CD9 expression may precede CD38 appearance. Cell sorting shows that colony-forming unit (CFU)-GEMM and CFU-GM are present in high proportions in this fraction and in the fraction with the lowest CD9 expression. Cells with the highest level of CD9 are committed to the B-lymphoid or megakaryocytic (MK) lineages, as shown by the co-expression of either CD19 or CD41/GPIIb and by their strong potential to give rise to CFU-MK. In liquid cultures, CD9(high)CD41(neg) cells give rise to cells with high CD41 expression as early as 2 days, and this was delayed by at least 3 to 4 days for the CD9(mid) cells; few CD41(high) cells could be detected in the CD9(low) cell culture, even after 6 days. Antibody ligation of cell surface CD9 increased the number of human CFU-MK progenitors and reduced the production of CD41(+) megakaryocytic cells in liquid culture. This was associated with a decreased expression of MK differentiation antigens and with an alteration of the membrane structure of MK cells. Altogether these data show a precise regulation of CD9 during hematopoiesis and suggest a role for this molecule in megakaryocytic differentiation, possibly by participation in membrane remodeling. (Blood. 2001;97:1982-1989)